Acrylic hot melt pressure sensitive adhesives and compounds of these are useful in the production of tapes, labels, and decals having superior weathering and solvent resistance to similar adhesives prepared from conventional styrene/diene block copolymers. The ability of these adhesives to be applied from the melt and their excellent creep resistance gives them significant performance and processing advantages over conventional solvent and emulsion based acrylic pressure sensitive adhesives.
Previous attempts to product hot melt acrylic pressure sensitive adhesives have involved the blending of high and low molecular weight acrylic polymers having carefully selected compatibility characteristics. These efforts have failed to produce an adhesive with the desired balance of adhesive and cohesive properties coupled with molten viscosities suitable for practical hot melt applications. Davis et al, in U.S. Pat. No. 3,925,282, discloses a system which incorporates a tertiary amine containing monomer in the tacky acrylic polymer followed by reaction of this polymer with a heavy metal containing compound. The use of heavy metals is undesirable, however, from the standpoint of product toxicity and no molten viscosities are given for their examples.
The preparation of macromolecular monomers and their copolymerization with acrylates is described in U.S. Pat. No. 3,786,116, by Milkovich et al. However, the patent does not teach the use of this technique for the preparation of the novel adhesive compositions described in the present invention.
Human et al, in European Patent Application Ser. No. 104,046, use the Milkovich copolymers in pressure sensitive adhesive compositions, but the viscosities of the compositions are too high for many hot melt applications.
Sliwka et al, in U.S. Pat. No. 3,558,746 utilized graft copolymers of styrene and and acrylates to obtain lower viscosity hot melt adhesives. The lower viscosity was obtained by graft structures and by keeping the molecular weights of the copolymers low by chain transfer and the use of higher initiator concentrations.